Fracture behavior of lithia disilicate- and leucite-based ceramics

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2 Figure 1 Diagram of the typical fracture surface features occurring in brittle materials. The regions are not drawn to scale. Adapted from Mecholsky (1993). 27 toughness or critical stress intensity factor ðK ICÞ can often be determined using the Griffith–Irwin equation: K IC ¼ Ys fc 1=2 ð1Þ where Y is a geometrical factor that accounts for the location and geometry of the critical flaw and type of loading, 7 s f is the stress at fracture, and c is the radius of an equivalent semicircular flaw for a semi-elliptical crack of semiminor axis ‘a’ and semimajor axis ‘b’ (Fig. 1). 8,9 Little information is available on the structural reliability of hot-pressed lithia disilicate-based and leucite-based ceramics, and the sensitivity of processing variables is unknown. This study was designed to characterize the fracture behavior and fracture toughness of a glass veneer (GV), a leucite-based ceramic, and two lithia disilicate-based ceramics using fractographic principles. The objective of this study was to test the hypothesis that variation in strength is associated with the variation in fracture toughness for the same surface preparation. Materials and methods The ceramic materials (E1, E2, ES, and GV) and firing procedures used for bar specimens Table 1 Firing temperatures ðTÞ and times ðtÞ for the four ceramics. Dental ceramics Starting T (8C) (25 £ 4 £ 1.2 mm) are summarized in Table 1. The GV specimens were fabricated using a metal mold and fired according to the manufacturer’s instructions (Table 1). The hot-pressed leucite-based core ceramic (E1) and the two hot-pressed lithia disilicate-based core ceramic specimens (E2 and ES) were prepared using the lost-wax technique. 10 After removal of the hot-pressed ceramic from the investment, the interaction layer was removed by grit blasting with 80 mm glass beads at a pressure of 2 bar (30 psi). The bar specimens were cleaned in 1% hydrofluoric acid (HF) for 30 min, grit blasted with 100 mm Al2O3 at a pressure of 2 bar, and polished through 1200 grit SiC metallographic paper to a thickness of 1.2 mm. All specimens were finished with 1 mm polishing alumina (Mark V Laboratory, East Granby, CT, USA) to the final dimensions (25 £ 4 £ 1.2 mm). They were ultrasonically cleaned in distilled water and steam cleaned using distilled water. Specimens were examined for flaws using light microscopy at 30 £ (microscope model SCW30L, Fisher Scientific, Thailand). Specimens with any obvious large flaws would be eliminated, if detected. However, no major flaws were detected. The specimens were stored for 48 h in distilled water at 37 8C and then subjected to four-point flexure loading (applied along rollers at 1/3 and 2/3 of the length of the bars) at Heating rate (8C/min) Firing T (8C) Holding t (min) Vacuum T on–off (8C) E1—IPS Empress core a (leucite-based ceramic) 700 60 1180 20 500–1180 E2—IPS Empress2 core a (lithia disilicate-based ceramic) 700 60 920 20 500–920 ES—Experimental core a (lithia disilicate-based ceramic) 700 60 910 15 500–910 GV—IPS Empress2 body a (amorphous glass) 403 60 800 2 450–799 a Ivoclar AG, Schaan, Liechtenstein. ARTICLE IN PRESS A. Della Bona et al.
a cross-head speed of 0.5 mm/min in a universal testing machine (Model 1125, Instron Corp., Canton, MA, USA) while immersed in distilled water. Note that any local residual stresses induced during preparation, i.e., around any flaws, were relieved by the storage under water because of slow crack growth in a conducive environment. An Isotemp Immersion Circulator (Model 730, Fisher Scientific, Pittsburgh, PA, USA), which was connected to the testing chamber, was used to maintain the water at 37 8C. 11 Failure loads were recorded and the flexural strength values were calculated using the following equation: s ¼ PL=wt 2 ð2Þ where P is the applied load at failure, L is the length of the outer (total) span, w is the width of the specimen, and t is the thickness of the specimen. 11,12 Fracture surfaces were examined using optical microscopy (OM) and scanning electron microscopy (SEM) to determine the mode of failure based on the fracture origin and fractographic principles. 3,13 – 15 In preparation for SEM (JSM 6400, Jeol Ltd, Tokyo, Japan) examination, the fracture surfaces were sputter-coated with gold–palladium for 3 min in a Hummer II Sputter Coater (21020, Technics Inc., Alexandria, VA, USA) at a current of 10 mA and a vacuum of 130 mTorr. The critical flaws were located and their size ðcÞ was determined using fractographic principles. The flaw size calculation ½c ¼ðabÞ 1=2 Š is the same as that used for an equivalent semicircular surface crack and corner crack. In the case of a corner crack, ‘c’ corresponds to the distance from the crack corner to the limit of the critical flaw-mirror region (Fig. 1). The mean crack size and strength values were used to calculate the fracture toughness ðK ICÞ via Eq. (1). The geometrical factor ðYÞ was calculated based on Randall’s 7 interpretation of Irwin’s 16 work: Y ¼½0:515Q 1=2 Š 21 ð3Þ Q ranged in value from 1.00 for long, shallow cracks ðY ¼ 1:94Þ to 2.46 for semicircular cracks ðY ¼ 1:24Þ: For a corner crack, the factor Y can be approximated by Y ¼ 1:12 2 2=p 1=2 ð4Þ where 1.12 2 represents the surface flaw corrections and 2=p 1=2 represents the correction for a pennyshaped embedded crack, i.e. Y ¼ 1:4. 17 Extra samples of each ceramic were fabricated and ground to a powder, mixed with an adhesive ARTICLE IN PRESS Fracture Behavior of Ceramics 3 (collodian–amyl acetate in a ratio of 1:7), and placed on a glass slide sample area. To identify the ceramic crystal phases, X-ray diffraction (XRD) was performed using a 3720 Phillips room temperature diffractometer (serial #1470, Phillips, Netherlands) and PW1877 automated powder diffraction software. A copper tube anode in a continuous scan from 3.01 to 89.978 (,3–908) with step size of 0.028 every 0.4 s was used at a voltage of 40 kV, and current of 20 nA. A slow scan from 10 to 408 was also used. The IPS Empress2 GV was analyzed for any crystal phase using a DRIFT (Diffuse Reflectance Infrared Fourier Transform, Spectra Tech Inc., Shelton, CT, USA) analyzer with a Gemini Stage (Nicolet Magna 760, Nicolet Inc., Madison, WI, USA) and OMNIC 4.1b software (Nicolet Inc., Madison, WI, USA). The following four powder samples were analyzed: (1) 0.5 g of KBr powder (Spectra Tech Inc., Shelton, CT, USA) for the background control; (2) 0.5 g of KBr powder mixed with 0.025 g of unfired incisal GV powder; (3) 0.5 g of KBr powder mixed with 0.025 g of unfired dentin GV powder; and (4) 0.5 g of KBr powder mixed with 0.025 g of fired dentin GV powder. Analysis was performed in 64 scans for four wave number resolutions. The data were corrected for H2O and CO2 interferences and results were recorded. Ceramic disks specimens of each material used in this part of the study (Table 1) were fabricated and polished through 1 mm alumina abrasive. The specimen thickness was measured using a caliper (Digimatic caliper, Mitutoyo Co., Kawasaki, Japan) and the weight was obtained using an analytical balance (Mettler H31, Mettler Instrument Corp., Hightstown, NJ, USA). The density ðrÞ was obtained using the Helium Pycnometer (Accupyc 1330, Micromeritics Instrument Co., Norcross, GA, USA) after calculating the volume. The elastic modulus ðEÞ and Poisson’s ratio ðnÞ were determined by means of ultrasonic waves and a computer program (ECALC and Sigview-F software, Nuson Inc., Boalsburg, PA, USA) based on a set of equations that uses the time of flight, density, and thickness. Piezoelectric transducers (Ultran Laboratories Inc., Boalsburg, PA, USA) and an ultrasonic pulse apparatus (Ultima 5100, Nuson Inc., Boalsburg, PA, USA) were used to determine the time of flight through the ceramic specimens. Longitudinal and shear (transverse) time of flight values were obtained and used to calculate n and E of the materials. 11 A hardness tester (Model MO Tukon Microhardness Tester, Wilson Instruments Inc., Binghampton, NY, USA) with a Vickers diamond was used to measure the hardness of these ceramic specimens. Each specimen was indented at four different locations under a load ðPÞ of
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Fracture behavior of lithia disilicate- and leucite-based ceramics

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